This invention concerns a quantum computer, that is a device for performing quantum computations. Recent progress in the theory of quantum computation, particularly the discovery of fast quantum algorithms, makes the development of such a device an important priority.
Finding an approach to quantum computation that fulfils the requirements has proved to be a formidable challenge. Nuclear spins have been incorporated into quantum computer proposals, because their lifetime can be at least six orders of magnitude greater than the time required to perform a logical operation on the spins.
The invention is a quantum computer, including:
A semiconductor substrate into which donor atoms are introduced to produce an array of donor nuclear spin electron systems having large electron wave functions at the nucleus of the donor atoms. Where the donor electrons (electrons weakly bound to the donor atom) only occupy the nondegenerate lowest spin energy level.
An insulating layer above the substrate.
Conducting A-gates on the insulating layer above respective donor atoms to control the strength of the hyperfine interactions between the donor electrons and the donor atoms"" nuclear spins, and hence the resonance frequency of the nuclear spins of the donor atoms.
Conducting J-gates on the insulating layer between A-gates to turn on and off electron mediated coupling between the nuclear spins of adjacent donor atoms.
Where, the nuclear spins of the donor atoms are the quantum states or xe2x80x9cqubitsxe2x80x9d in which binary information is stored and manipulated by selective application of voltage to the A- and J-gates and selective application of alternating magnetic field to the substrate.
A cooling means may be required to maintain the substrate cooled to a temperature sufficiently low, and a source of constant magnetic field having sufficient strength to break the two-fold spin degeneracy of the bound state of the electron at the donor may also be required. The combination of cooling and magnetic field may be required to ensure the electrons only occupy the nondegenerate lowest spin energy level.
The device may also incorporate a source of alternating magnetic field of sufficient force to flip the nuclear spin of donor atoms resonant with the field, and means may be provided to selectively apply the alternating magnetic field to the substrate.
In addition the device may include means to selectively apply voltage to the A-gates and J-gates.
The invention takes advantage of the fact that an electron is sensitive to externally applied electric fields. As a result the hyperfine interaction between an electron spin and the spin of the atomic nucleus, and the interaction between an electron and the nuclear spins of two atomic nuclei (that is electron mediated or indirect nuclear spin coupling) can be controlled electronically by voltages applied to gates on a semiconductor device in the presence of an alternating magnetic field. The invention uses these effects to externally manipulate the nuclear spin dynamics of donor atoms in a semiconductor for quantum computation.
In such a device the lifetime of the quantum states (or qubits) operated on during the computation must exceed the duration of the computation, otherwise the coherent state within the computer upon which quantum algorithms rely will be destroyed. The conditions required for electron-coupled nuclear spin computation and single nuclear spin detection can arise if the nuclear spin is located on a positively charged donor in a semiconductor host. The electron wave function is then concentrated at the donor nucleus (for s-orbitals and energy bands composed primarily of them), yielding a large hyperfine interaction energy. For shallow level donors, however, the electron wave function extends tens or hundreds of xc3x85 away from the donor nucleus, allowing electron-mediated nuclear spin coupling to occur over comparable distances.
An important requirement for a quantum computer is to isolate the qubits from any degrees of freedom that may interact with and xe2x80x9cdecoherexe2x80x9d the qubits. If the qubits are spins on a donor in a semiconductor, then nuclear spins in the host are a large reservoir with which the donor spins can interact. Consequently, the host should contain only nuclei with spin I=0. This requirement eliminates all III-V semiconductors as host candidates, since none of their constituent elements possess stable I=0 isotopes. Group IV semiconductors are composed primarily of I=0 isotopes and may be purified to contain only I=0 isotopes. Because of the advanced state of Si materials technology and the tremendous effort currently underway in Si nanofabrication, Si is an attractive choice for the semiconductor host.
The only I=xc2xd shallow (Group V) donor in Si is 31P. The Si:31P system was exhaustively studied forty years ago by Feher in the first electron-nuclear double resonance experiments. At sufficiently low 31p concentrations at temperature T=1.5 K, Feher observed that the electron relaxation time was thousands of seconds and the 31P nuclear relaxation time exceeded 10 hours. At millikelvin temperatures the phonon limited 31P relaxation time may be of order 1018 seconds, making this system ideal for quantum computation.
The A- and J-gates may be formed from metallic strips patterned on the surface of the insulating layer. A step in the insulating layer over which the gates cross may serve to localise the gates electric fields in the vicinity of the donor atoms.
In operation the temperature of the quantum computer may be below 100 millikelvin (mK) and will typically be in the region of 50 mK. The process of quantum computation is non-dissipative, and consequently low temperatures can be maintained during computation with comparative ease. Dissipation will arise external to the computer from gate biasing and from eddy currents caused by the alternating magnetic field, and during polarisation and detection of nuclear spins at the beginning and end of the computation. These effects will determine the minimum operable temperature of the computer.
The constant magnetic field may be required to be of the order of 2 Tesla. Such powerful magnetic fields may be generated from superconductors.
The extreme temperatures and magnetic fields required impose some restrictions on the availability and portability of the quantum computing device outside of a laboratory. However, the high level of access to a computer situated remotely in a laboratory, for instance through use of the internet, may overcome any inconvenience arising from its remoteness. It is also feasible that the device could be utilised as a network server for personal computers, in which case the server may have a local cooling system and the personal computers may operate at room temperature.
The initial state of the computer must be accurately set and the result of the computation accurately measured. Electron devices may be provided to set the initial state and read output from the quantum computer. These devices polarize and measure nuclear spins. For example, the electron device may modulate the movement of a single electron, or a current of electrons, according to the state of a single nuclear spin. These devices will typically be provided at the edge of the array.
An electron device for polarizing and measuring nuclear spins may, comprise:
A semiconductor substrate into which at least one donor atom is introduced to produce a donor nuclear spin electron system having large electron wave functions at the nucleus of the donor atom.
An insulating layer above the substrate.
A conducting A-gate on the insulating layer above the donor atom to control the energy of the bound electron state at the donor.
A conducting E-gate on the insulating layer on either side of the A-gate to pull electrons into the vicinity of the donor.
Where in use, the gates are biased so that, if the transition is allowed, one or more electrons can interact with the donor state.
In a further aspect, the invention is a method of initializing the quantum computer, comprising the following steps:
biasing the gates so that, if the nuclear spin of a donor is in a first state no transition is allowed, but if the nuclear spin is in a second state transition is allowed, and one or more electrons can interact with the donor state to change the nuclear spin to the first state; and
continuing the process until all the donors are in the first state.
In a further aspect, the invention is a method of measuring nuclear spins in the quantum computer, comprising the following steps:
biasing the gates so that, if the nuclear spin of a donor is in a first state no transition is allowed, but if the nuclear spin is in a second state transition is allowed, and one or more electrons can interact with the donor state to change the nuclear spin to the first state; and
detecting the movement of the one or more electrons to determine the state of the respective donors.